1. Field of the invention
The present invention is generally concerned with torsional damper devices comprising at least two coaxial parts disposed to rotate relative to one another within defined limits of relative angular displacement against elastic means. The elastic means, usually called circumferentially acting elastic means, is adapted to be operative circumferentially between the two coaxial parts for part at least of such relative angular displacement. Namely, a first part which comprises a hub and a flange, usually called the hub flange, which forms an annular part around the hub and which, for at least one circumferential direction, is constrained to rotate with the hub. A second part which comprises at least one flange, usually called a guide ring and forms an annular part around the hub, but which is free to rotate relative to the hub in both circumferential directions.
2. Description of the prior art
As is known, this type of torsional damper is normally incorporated in the design of a clutch disk, particularly for automobile vehicles. Its rotary parts which comprise at least one guide ring carrying a friction disk designed to be constrained to rotate with a first shaft, a driving shaft or the motor output shaft in the case of an automobile vehicle. The other of these rotary parts is adapted to be constrained to rotate through its hub with a second shaft, a driven shaft or the gearbox input shaft in the case of an automobile vehicle.
In this way it is possible with advantage to procure regulated transmission of rotational torque from one of these shafts to the other. It provides filtering of vibrations which may arise at any point in the kinematic system in which the shafts are incorporated, extending from the motor to the wheel shafts in the case of an automobile vehicle.
In practice two guide rings are provided, parallel to and axially spaced from one another. One guide ring on each side of the hub flange and linked to one another by axial spacers which pass through said hub flange by means of openings provided in it for this purpose.
One problem to be solved in implementing torsional damper devices of this kind results from the need to reconcile two apparently contradictory requirements.
The first of these requirements relates to the fact that to minimize or even reduce certain sources of noise. More precisely the noise commonly called "trash" noise whish arises when the driver of the vehicle concerned depresses or releases the accelerator pedal and the gears in the gearbox of the vehicle impact on one another under load. It appears desirable, as is confirmed by experience, that the circumferentially acting elastic means, disposed between the two coaxial parts of a torsional damper device of this kind which are the first to come into action, features a relatively low stiffness. In practice it is not of more than 1 m.daN per degree.
As a corollary to this, a second requirement which normally has to be met, and which is in this instance imperative, is that at the end of angular displacement between said rotary parts, the maximum permissible torque is greater than the most usual maximum motor torque values. These are frequently of the order of 15 to 17 m.daN.
For safety reasons, it is therefore desirable that the maximum torque that can be transmitted between the two coaxial parts of a torsional damper device of the kind in question should be of the order of 20 m.daN, for example. In torsional damper devices of the kind in question known at present, the maximum angular displacement possible between said coaxial parts is limited. This angular displacement is limited by the circumferential extent that can be given to the openings formed in the hub flange for the axial spacers coupling together the two guide rings to pass through.
Beyond a certain circumferential dimension, these openings inevitably reduce to an unacceptable, degree the mechanical strength of the hub flange, reducing the service life of the device.
In practice, in torsional damper devices of the kind in question known at present, the angular displacement between the two coaxial parts is more often 10 degrees or less. Thus if, in order to minimize "trash" noise, there are employed between these two parts circumferentially acting elastic means which have only moderate stiffness, of the order of 1 m.daN per degree. The maximum permissible torque at the end of their angular displacement is at most of the order of 10 m.daN, which is insufficient.
A general object of the present invention is an arrangement whereby this difficulty may be overcome and hence the contradictory requirements reconciled.